The present invention relates to an improved level shift circuit for shifting an amplitude center level of an A.C. input signal.
When a power supply voltage changes in amplifier circuitry connected to a single power supply, it becomes necessary to shift the amplitude center level of an input signal. In portable devices or the like, where a lithium-ion battery of 3.6 volts is employed, the battery power is used at their control stage after having been lowered via a regulator to 3.0 volts, while the 3.6-volt battery power is directly used at their power stage without being passed through the regulator. In such cases, the amplitude center level of an input signal is controlled to amount to 1.5 volts in an amplifier circuit of the control stage, and it is controlled to amount to 1.8 volts in an amplifier circuit of the power stage. Namely, in such amplifier circuitry, the amplitude center level has to be raised to 1.8 volts at the power stage by means of a level shift circuit.
In FIG. 6, there is shown an example of a conventionally-known level shift circuit of the above-discussed type. In the circuit of FIG. 6, an amplitude center level Vref1 (see a waveform W1 on the left of the figure) is applied to an amplifier circuit 4 that comprises resistors 1 and 2 and an operational amplifier 3, and an amplified output of the amplifier circuit 4 is then level-shifted, via a D.C.-cutting capacitor 5 and another amplifier circuit 9, into a signal having an amplitude center level Vref2. Here, the D.C.-cutting capacitor 5 is provided outside an integrated circuit chip having the amplifier circuit 4 fabricated thereon, and the amplified output of the amplifier circuit 4 is passed through the capacitor 5 and then applied to an input terminal of the amplifier circuit 9 comprising resistors 6 and 7 and an operational amplifier 8. Reference voltage Vref2 is applied to a noninverted input terminal of the operational amplifier 8. In this way, the amplifier circuit 9 can output an amplified signal having the amplitude center level Vref2 as illustrated on the right of the figure by reference character W2.
However, the above-discussed conventional level shift circuit indispensably requires the external capacitor 5 of a relatively great capacity and can not be implemented by the integrated circuit chip alone. Accordingly, the integrated circuit chip must have an extra connecting terminal for the capacitor 5, which undesirably increases the number of necessary manufacturing steps. Further, because the conventional level shift circuit requires two different reference voltages Vref1 and Vref2 as noted above, it needs a complicated reference-voltage generating circuit.
In view of the foregoing, it is an object of the present invention to provide an improved level shift circuit which requires no external capacitor and requires only one reference voltage.
In order to accomplish the above-mentioned object, the present invention provides a level shift circuit comprising: an input resistor, an input signal being applied to one end of the input resistor; an amplifier having an input terminal connected with the other end of the input resistor; a feedback resistor connected between the input terminal and output terminal of the amplifier; and a level-shifting resistor connected between the input terminal of the amplifier and a predetermined constant voltage point.
In one embodiment of the present invention, the level-shifting resistor may be replaced with a constant-current power supply.
In another embodiment of the present invention, the level-shifting resistor may be replaced with a resistor circuit that includes a plurality of series-connected resistors, and a switching section connected between respective connection points of the series-connected resistors and a predetermined constant voltage point.